1. Field of the Invention
The present invention generally relates to a transformer inrush current elimination technique for suppressing an inrush current generated upon connecting a transformer to a three-phase power supply, and more particularly, to an inrush current elimination technique using a controlled closing method for suppressing an inrush current transited in a three-phase transformer having a primary winding of a star connection type with a grounding neutral and a secondary or tertiary winding of a triangle connection type.
2. Description of the Prior Art
Conventionally, in, order for eliminating an inrush current in a transformer, there has been taught, for example, a controlled closing method using a circuit breaker connected in a specific phase of a three-phase power supply. The following description relates to a transformer inrush current elimination system of a controlled closing method.
In a conventional transformer inrush current elimination system using this method, a step-down means for lowering a phase voltage of a three-phase transformer is used so that the phase voltage is made suitable for an input signal of an optimum making (or energization) phase calculation device serving as an electronic equipment. That is, the step-down means lowers a phase voltage of each phase, which is generated upon de-energization (i.e., shut-down) of the three-phase transformer from the power supply and changes in a transient manner to finally become zero. By using these three step-down phase voltages as the input signals of the optimum making phase calculation device, the input voltages are subjected to a time-wise integrating operation by a residual flux calculation means to thereby calculate a residual flux inside an core of the three-phase transformer. Then, a making phase calculation means calculates three optimum making phases that are respectively different without causing an inrush current, by using a formula derived from a relational expression among a flux at the time of the making (energization), a making phase and the calculated residual flux. These calculation results are used as output signals of the optimum making phase calculation device, and thus, the output signals are used as making phase signals of a controlled switching unit of a circuit breaker. The making phase signals are individually inputted for the respective phases in the circuit breaker. See, for example, Patent Document 1. Japanese Patent No. 2685574 (Specification pages 4–10, FIGS. 1 to 10).
Moreover, in another conventional transformer inrush current elimination system of this type, the disclosure takes into consideration in practical use, regarding the points of mechanical closing time deviations of the circuit breaker, influences of pre-arc of the circuit breaker, and the like. See, for example, Non-Patent Document 1: “Elimination of Transformer Inrush Current by Controlled Switching” (IEEE TRANSACTIONS ON POWER DELIVERY) written by John. H. Brunke and Klaus. J. Frohlich), (U.S.) p. 276–285, Second Issue, Vol. 16, April 2001.
However, in the conventional techniques described in Patent Document 1, no consideration is given to mechanical closing time deviations and influences of pre-arc of the circuit breaker. Therefore, the phase input is sometimes carried out at a point offset from an optimum making phase, and in such a case, there has been a problem that an excessive inrush current tends to occur.
In addition, with respect to the optimum making phases of the rest two phases after the first phase has been inputted, the disclosure of Patent Document 1 takes no consideration of damping of DC flux components of a transformer core, which is caused by a triangle connection of a secondary or tertiary winding. Therefore, the optimum making phases of the rest two phases are not appropriate, resulting in occurrence of an excessive inrush current.
Moreover, in the conventional technique described in Non-Patent Document 2, there is no concrete description of a specific solution for solving a problem of mechanical closing time deviations and influences of pre-arc in the circuit breaker.